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  • By Mark Meshulam

    Chicago Ground Hog Day 2011 Blizzard – Envision the Wind

    On February 1, 2011 at about 3pm, a somewhat rare occurrence took place in Chicago. A real-life blizzard! It continued for 20 hours.

    Contrary to the misconceptions of some, a blizzard is not distinguished by a large volume of snow, but rather by an intense and pervasive wind.

    In order to achieve the coveted blizzard status, a storm must have winds in excess of 35 mph for a duration of 3 hours or more. This results in low visibility, 1/4 mile or less.

    One of the problems we have in trying to understand the wind is that it is usually invisible. A blizzard makes the wind visible. The millions of tiny snowflakes move along with the wind, expressing the wind’s every huff, puff and nuance.

    Now it’s time to don our special glasses and understand exactly what we are seeing when we see snow blown around by the wind. In real time and 3D, we are actually seeing the three variables that we love to speak about when wind is concerned: direction, speed and pressure. The wind speed and direction are easily seen when snowflakes are suspended in the air:

    Snowflakes suspended in the air allow us to visualize wind speed and direction

    Less intuitive is the idea that we can also see the wind pressure, too. It’s easy when you know how. When you see a group of snowflakes crowded together, they are in an area of higher pressure. In areas where they are spaced far apart, the pressure is relatively lower. The snowflakes can be thought of as very large air molecules. How do we know this? Elementary, dear Watson.

    If you begin with an assumption that in a neutral (no wind) state, snow particles would be evenly distributed in the air as they fall, then we take any cubic foot of that snowy air and compress it (this requires increased pressure on all sides of the cube) to half its volume, the pressure inside the shrunken cube would now be greater. The same number of snowflakes would occupy much less space, and would now be closer together. This closeness would form a cloudlike appearance as seen in fast-moving snow cloud in the image above.

    If you took another cubic foot of air and stretched it to double its volume (this requires reduced pressure to pull at at the boundary of the cube) creating a reduced pressure inside the cube, the visual impression of the snowflakes would be more dispersed. You might not even see the snowflakes.

    Densest portions of fast moving snow cloud are areas of highest pressure

    Now we consider the collision of two winds such as can easily happen when winds are whipping around our buildings. The colliding winds can increase pressure at the point of collision, similar to the denting of fenders a car crash. Soon that increased pressure finds a way to dissipate in the path not only of least resistance, but where the stronger of the winds has its way. In the clip below, one “wind” is supplied by a snowblower that propels snowy air into the path of a crosswind. The resultant patterns were interesting and hypnotic.

    Collision of wind forces is resolved when stronger wind prevails

    Unhindered wind creates turbulence along the surface of the ground, even without obstacles. The first part of this action is the creation of a negative pressure at the surface of the ground. This pulls snow (and air) from the ground upward into the windflow. The raised snow is impacted by the rushing wind which creates a momentary higher pressure at the collision, causing some of the snowy air to roll back down toward the ground. A rolling motion is set up wherein an infinite number of horizontally rolling vortices progress along the ground surface.

    A carpet of moving snow at ground level shows the rolling motion of an infinite number of vortices

    Air is a fluid and can behave just like water. In the clip below, the wind intensifies as it passes over the top of a ridge. This intensification could be thought of as a funneling process. The wind became compressed as it encountered the ridge because there was less space for the wind to travel within. As a result, the speed and pressure increased.

    Wind speed and pressure increase as wind is funneled over a ridge

    If you have been keeping up with these articles, and especially this one, you will remember that the strongest forces acting upon our buildings is the negative pressure experienced on the leeward side (the side away from the wind) of the structure. This same pressure lifts airplanes and propels sailboats. It also digs holes in the snow at the base of trees as seen below.

    Negative pressure at leeward side of ground recess pulls yet more snow from the recess

    In the clip above, wind skims along the ground’s surface much as it does when it encounters the flat face of a building. When the wind encounters the depression in the snow, it continues forcefully in its original direction, however a negative pressure is created in the recess, drawing more and more snow out of the recess. This is yet another example of a negative pressure on the leeward side of an object. This is why a known scenario for wind damage to buildings is when glass is sucked out of the building at the building corners in high winds.

    Interestingly, the vertical face of the recess is curved, reflecting the path and intensity of the negative pressure vortex created in the recess.

    I am sure my fellow Chicagoans join me in hoping for many more blizzards so that we can learn more about the wind.

    Is the wind affecting your building?

    Mark Meshulam braving the elements so that you can sit in your cushy easy chair with a mug of hot chocolate and think about how windy and cold it was.

    Mark Meshulam braving the elements so that you can sit in your cushy easy chair with a mug of hot chocolate and think about how windy and cold it was.

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